1. Field of the Invention
The present invention relates to an image forming apparatus and method for developing an electrostatic latent image formed on the surface of a photoreceptor using a developer carried by a developer support with a grooved surface.
2. Description of Related Art
In the image forming apparatus that employs electrophotography, an exposure device emits an optical beam modulated with image data to the surface of a charged photoreceptor. As a result, an electrostatic latent image is formed on the surface of the photoreceptor. Moreover, a development device holds a binary developer composed of a magnetic carrier and a non-magnetic toner, and a development roller mounted therein carries the binary developer to a position to face the photoreceptor. At this time, a developing bias voltage is applied to the development roller. As a result, the electrostatic latent image is developed by the binary developer, resulting in a visually recognizable toner image formed on the surface of the photoreceptor. Such a toner image is formed for each of the four colors yellow, magenta, cyan, and black, for example. The toner images of these colors are transferred and overlaid on one another as a composite image on an intermediate transfer belt.
Incidentally, printing paper, which is a typical example of a recording medium, is introduced to a nip (i.e., secondary transfer portion) between the intermediate transfer belt and a secondary transfer roller. The composite image on the intermediate transfer belt is subjected to secondary transfer onto the introduced printing paper by means of an electric field from the secondary transfer roller. Thereafter, the printing paper with the composite image transferred thereon is introduced to a fusing unit, and the fusing unit heats and fixes the toner. Subsequently, a print is ejected onto an output tray.
The force to carry the binary developer is obtained from the frictional resistance of the development roller surface. To maintain this carrying force, it is desirable for the frictional resistance not to be degraded over long-term use. However, in the development process, the development roller and the developer repeatedly collide with each other. Accordingly, in the case where aluminum, which is liable to deteriorate, is used as the material of the development roller, simply roughening the development roller surface by blasting or suchlike does not keep the frictional resistance from degrading over time, thereby failing to provide a stable force to carry the binary developer. In view of this problem, there has been proposed an approach in which the surface of a development roller is grooved with a predetermined pitch (e.g., see Japanese Patent Laid-Open Publication No. 2011-100145). For example, forming grooves with a pitch of about 1 millimeter [mm] can reduce degradation of the frictional resistance over time, making it possible to maintain the force to carry the binary developer for a longer period of time.
However, the amount of binary developer supported on the grooved surface of the development roller is lesser on ungrooved portions (land portions) than in grooves (depression portions), resulting in a problem with unstable density of an image formed on a print. Uneven density of an image is very noticeable particularly in highlighted areas, which are highly visible portions of the image.